Custom shields can be plugged into the header rows on the Mayfly. The headers break out all of the pins that are available, including ones that aren’t accessible though the Grove connectors on the Mayfly.
A small board can break out the serial UART pins to a Grove socket. Pads under the board allow you to select whether it’s Port 0 or 1, and what voltage signal is available on the V pin of the Grove socket. A 2-port UART board is also available.